Prior art techniques for testing of communications networks generally fall into two categories: in-situ and ex-situ. Existing in-situ testing methods are either invasive or non-invasive. One invasive in-situ technique involves performing a precise bit error ratio (BER) test to assess gross functionality as well as parametric performance. Such a technique is described in International Publication No. WO 01/073465 A3 (International Application No. PCT/US01/09805), filed Mar. 26, 2001, published Oct. 4, 2001, and entitled “APPARATUS AND METHOD FOR BUILT-IN SELF-TEST OF A DATA COMMUNICATIONS SYSTEM.” A problem with this technique is that it relies on sending and receiving known data, such as pseudorandom bit streams, to measure BER. Thus, the test is invasive since the normal operation of the communications link is interrupted.
Existing non-invasive in-situ techniques also involve measuring BER. Such non-invasive in-situ techniques are described in U.S. Pat. No. 6,310,911 B1, filed Feb. 11, 1998, issued Oct. 30, 2001, and entitled “METHOD OF DETECTING SIGNAL DEGRADATION FAULT CONDITIONS WITHIN SONET AND SDH SIGNALS,” and the American National Standards Institute (ANSI) Synchronous Optical Network (SONET) standard (available from the International Engineering Consortium at http://www.iec.org/online/tutorials/sonet/). BER is measured by comparing the known bits in coding overhead to the values that arrive at a given receiver. If the received values deviate from the expected values, there are errors. A problem with this method of measuring BER is that it only provides an eye-center BER without reference to BER vs. phase (i.e. jitter). Any parametric measurements using BER vs. phase would be representative only of the performance of the clock recovery circuit, which would mask the performance of the optical link, where degradation is most likely.
Ex-situ techniques involve using test equipment hooked directly to a port being measured. The test equipment varies from limited, small, and inexpensive to large, expensive, and highly capable. The small, simple testers perform measurements ranging from BER and protocol analysis to optical time domain reflectometry (OTDR), but none of these testers measure alternating current (AC) waveform properties. The larger, more sophisticated test equipment category contains virtually every type of measurement capability, including measuring AC waveform quality, but can be quite expensive and cumbersome.
It would be desirable to provide a network monitoring system that is both ongoing (non-invasive) and precise to predict when a network component will fail.